1. Field of the Invention
The present invention relates to a propelling device for assisting insertion of a tip end portion of an endoscope.
2. Description of the Related Art
Endoscopes are widely used for examination of curved alimentary canals like the large intestine or the small intestine. The endoscopes are constituted by a manipulating part and a flexible insertion part. The insertion part is inserted into an alimentary canal. Observation, diagnosis and medical treatment of the inner wall surface of an alimentary canal are performed (for example, refer to JP 2005-253892 A). In this case, if the alimentary canal is a sigmoid colon that is curved in a complicated manner and moves relatively freely, a high level of skill is required to advance the insertion part to the depths of the sigmoid colon. For this reason, an endoscope whose insertion part can be easily advanced to the depths even within an alimentary canal, such as the sigmoid colon which is curved intricately, is needed.
In recent years, a propelling device, which is attached to the tip end side of the insertion part and propels the insertion part within an alimentary canal has been developed (for example, refer to JP 2009-513250 A). A traveling body (circulating moving body) of this propelling device is attached to an external cylinder mounted on the tip end side of the insertion part, and the traveling body is made to travel in a circulating manner in a state where an outside of the traveling body is brought into contact with the inner wall of the alimentary canal, and thereby the tip end side of the insertion part is moved within alimentary canal by the friction generated between the outside of the traveling body and the inner wall of the alimentary canal. In this propelling device, the traveling body is pressed against a driving gear and pinched by a pair of driven rollers, and the driving gear is rotated with a driving force from the outside, whereby the rotation of the driving gear is transmitted to the traveling body to make the traveling body travel. There is also described an another configuration in which a plurality of driving sets each including a pair of driven rollers and a driving gear are provided side by side in the traveling direction of the traveling body such that the traveling body is made to travel.                One technique for transmitting a large driving force to the traveling body is to increase the contact area between the driving gear and the traveling body. Accordingly, it is useful to increase the number of the driving gears by providing a plurality of driving sets side by side in the traveling direction of the traveling body as described in JP 2009-513250 A. However, on the other hand, since a pair of driven rollers are needed for every driving gear, a problem occurs in that the total length of the propelling device becomes large.        
Thus, the number of the driven rollers can be reduced by arranging two driving gears in proximity with each other, and arranging one driven roller so as to face a portion between the driving gears, such that the traveling body is pinched between the one driven roller and two driving gears. However, in the case of adopting such a configuration, there is a concern that due to an assembling error of the driving gears and the driven roller, respective gaps between the respective driving gears and the driven roller may become non-uniform, and as a result, the respective pinching forces may become non-uniform. If the respective pinching forces become non-uniform, not only a driving force is no longer transmitted to the traveling body effectively, but also tensioning or loosening occurs on the traveling body between the respective driving gears, which causes a problem that the traveling body, the driving gears, the driven roller, and the like are damaged.